In accordance with the recent development of data communication techniques, there are cell phones provided with a function for transmitting and receiving e-mail or PDA (personal digital assistant) devices equipped with a letter input device with a data communication function.
FIG. 1 illustrates a front view of a conventional general purpose cell phone. This cell phone is comprised of a display 02, an antenna 03, a loudspeaker 04, a microphone 05, a power switch 10, various mode switch keys 11, a joy key 12, letter keys 17, and a main body 16.
As shown in FIG. 1, the cell phone is generally equipped with about twelve letter keys, about four various mode conversion keys, and one joy key so that letters can be inputted through a same flat surface as a display (information display section).
A PDA device also has a similar number of keys on the same flat surface as a display. Some PDA devices have fewer keys than those of cell phones while other PDA devices have fifty keys or more as the key board of a personal computer does.
FIG. 2 illustrates a front view of a letter input device for a conventional folding type cell phone. This letter input device has at the center section thereof a hinge mechanism 14 by which the device is configured to be folded so that a display and a key board section oppose to each other when the device is not used. Such a mechanism allows the device to be smaller and thus to be conveniently carried.
A cell phone and a PDA device both have letter keys on a same flat surface as a display. Alternatively, the foldable type device, as shown in FIG. 2, in which one flat surface having the letter keys and another flat surface having the display can be folded when the device is not used has an angle of gradient between the display and the key board section as in a notebook computer. Both the devices shown in FIGS. 1 and 2, however, have the key board section on the flat surface only which can be seen in front of a user.
When letters are inputted using a cell phone, a right-handed user holds the cell phone with the right hand to move the display to the front of the user's eyes, thereby moving the right hand thumb to push one key at a time, as shown in FIG. 3.
When the user holds the cell phone by the right hand to operate the right hand thumb to push the keys for letter input, the user's left hand is free and thus can be arbitrarily used for other tasks (e.g., hanging on a strap in a train or railcar or holding a bag).
Of the PDA devices, small types thereof which can be operated with one hand are used in a similar way as for a cell phone. When a left-handed user uses such a device, then the user uses the hand fingers opposite to those used by the right-handed user to push the keys.
A large PDA device is placed on a desk so that the fingers of both hands are used to input letters, as in the case of the key board of a personal computer. This prevents one hand from arbitrarily being used for other tasks and also requires the use of the desk or knees. For a cell phone or a small PDA device which is held by one hand to be operated with the fingers of the same hand, such a device requires the one hand holding the device to use the thumb to sequentially operate about twelve keys on the key board section in front of the user for the letter input. Thus, the rate at which text is prepared is determined by the rate at which the thumb pushes each of the keys for the letter input.
The key board section provided on the front surface of the cell phone has about twelve keys to which twenty-six alphabet letters A to Z and ten numeric characters are assigned. On average, one key is assigned with three alphabetical letters and one numeric character.
A letter input rate when only the thumb is used to input letters is about one-twentieth slower than that obtained when ten fingers of both hands are used for inputting letters as in a personal computer. Such a slow speed prevents even a technique enabling a faster data communication rate from being sufficiently utilized because the letter input rate is not improved. This is due to the fact that only one thumb of one hand is used, which requires the thumb to manage too many keys and thus requires the thumb to move in an excessively broad range. This is also due to the fact that the excessively broad range in which the thumb must move leads to an extreme difficulty of touch typing (i.e., an operation in which a user presses keys without looking at the keys the user pushes while looking at a display on which the letters corresponding to the pushed keys are displayed). The use of only the thumb for letter input also requires the user to alternately switch the user's field of view to the display and the key board on the front surface having twelve keys, which causes not only a slower input rate but also an increased amount of input errors.
Thus, it is desired to allow cell phones and small PDA devices to have a faster letter input rate. It is also desired to provide such a device that does not always require only one hand to input letters when using a cell phone in the case where both hands are free so that both hands are allowed to be used for the letter input, thereby providing a faster letter input rate. Such a device is desired by all users.
If the letter input rate of a cell phone can be as fast as that of a personal computer on a desk, then a demand for business communication through the cell phone can be drastically expanded.
At present, a cell phone or a PDA device having such letter keys is not marketed. Under the present situation, the following patents are published as the general technical level of this field:    JP2001-022501;    JP09-083402;    JP2000-59483;    JP2000-151774;    JP2001-060999;    EP 104709;    JP2001-117709;    JP10-224288;    JP06-274257;    U.S. Pat. No. 4,791,408;    U.S. Pat. No. 4,360,892;    U.S. Pat. No. 5,281,966;    JP2000-244623; and    JP2000-27653.
All of these patents only attempt to provide a faster letter input rate by providing five keys on a side face so that up to five fingers of one hand can be used for the letter input or only try to eliminate the limitation of the number of keys due to the small space of the front surface of a cell phone when information is accessed using the cell phone by providing about four auxiliary keys on the side face so that about twelve keys on the front surface can play an increased number of roles.
There are a number of other publications of patent applications which try to provide a faster letter input rate of a cell phone by using only about twelve keys on a front surface by the contrivance of software. None of the patent applications, however, are effective.
At present, such techniques as suggested in the above publications are not practically marketed. The reason is that the letter input rate of a cell phone having the above-described side face keys and/or side face auxiliary keys is not remarkably faster than that of a currently-marketed product having a keyboard on the front surface thereof.
It should be noted that an external key board is a hot-selling product by which a cell phone and a JIS key board are connected through a cable as disclosed in JP2000-244623.
JP2000-27653 discloses a technique wherein the little fingers of both hands are fixed to a PDA device to hold it. The published patent applications described above indicate the difficulty to input letters with the current cell phones.